At GitHub I already expressed my gratitude to the Dev @erosman, so my review below is for potential new FireMonkey' users.

It is an add-on for smart users!
FireMonkey easily replaces several other add-ons, using just 20% of the resources (RAM, CPU etc). The improvements in browser performance, security and privacy are visible.
In addition, FireMonkey is the only add-on (in its class) really Open Source, with the express commitment of the Dev not to invade privacy.
Another advantage is that FireMonkey uses the latest newest API.

Those users who don't care about performance, security, privacy, etc., and want lights, garlands, colors, glitter and fancy stuff ... FireMonkey may not be for them.

Some of the other similar add-ons have many automated functions, while in FireMonkey some are manual. At first I felt intimidated. But in less than 2 days of use, without problems I got used to everything. Today with few days of use, it seems like I have FireMonkey for years. I had practically no problems in the transition and migration of all my other add-ons/scripts (CSS & JS).
FireMonkey is a minimalist add-on, but totally powerful. Virtually any difficulty in the transition of other add-ons can be solved in FireMonkey (or at least almost solved). By the way, every excellent add-on requires a small investment of time at the beginning. But it's just a one-time-task/job.

The Dev (@erosman) is very responsive, and replies very promptly at GitHub. The few problems I found, the Dev solved them on the same day.

FireMonkey is not perfect, and it will undoubtedly have a path of evolution. But today, I can't imagine going back to use all the add-ons FireMonkey replaced.

Do you have doubts? Fears? Take a simple test!... it'll be difficult to repent.

Here I am giving 5 stars because it is the maximum, but FireMonkey deserves all the possible stars.

Attached a review from GHacks: https://www.ghacks.net/2019/09/10/firemonkey-uses-firefoxs-official-api-for-userscripts-and-userstyles/